If you're looking forward to lots of outings with your partner, take a look at these details about year-round temperatures in Yankton: The hottest months are usually July and August, with an average temperature of 22°C, while the coldest months are January and February, with an average of -3°C. The snowiest months in Yankton are January, December, October and March, with each month seeing an average of 20 cm of snowfall.